Conventional glass finishing technology has largely been developed for thick sheet glass manufacturing techniques, primarily for the automotive industry. These finishing techniques typically employ a mechanical scoring and breaking process in which a diamond or carbide scoring wheel is dragged across the glass surface to mechanically score the glass sheet, after which the glass sheet is bent along this score line to break the glass sheet, thereby forming a break edge. The edges of the glass sheet are then typically rounded using a metal grinding wheel having a radiused groove on its outer periphery, with diamond particles embedded in the radiused groove. By moving the glass sheet against this radiused groove and rotating the diamond wheel at a high rpm, a radius is literally ground into the edge of the glass sheet. Such methods are commonly employed, for example, to make the rounded edges on automotive windows, and for this purpose these methods are completely satisfactory.
Flat panel display substrates require much thinner glass sheets than are employed in the automotive industry, and have product requirements and robustness requirements which are significantly different from these products. Mechanical scoring and breaking techniques commonly result in lateral cracks about 100 to 150 microns long which emanate from the score wheel cutting line. Previously, these lateral cracks would have to be removed by diamond wheel grinding methods, discussed above. However, to do so involved removal of about 100 to 200 microns or more of the glass edge. Consequently, the diamond wheel grinding step (and also the mechanical scoring step) created an enormous amount of debris and particles.
In addition, in spite of repeated washing steps, particles generated during finishing continue to be a problem. For example, in some cases particle counts of substrates prior to shipping were actually lower than subsequent particle counts taken after a period of time. This is because the grinding of the glass sheets resulted in chips, checks, and subsurface fractures along the edge of the ground surface, all of which serve as receptacles for particles. These particles subsequently would break loose at a later time, causing contamination, scratches, and sometimes act as a break source in later processing. Consequently, such ground surfaces are "active", meaning subject to expelling particles with environmental forces, such as temperature and humidity. The present invention relates to methods for reducing these "lateral cracks" and "micro-checking" caused by grinding, thereby forming a glass sheet having edges which are more "inactive".
Laser scoring techniques can greatly reduce lateral cracking due to conventional mechanical scoring. Previously, such laser scoring methods were thought to be too slow and not suitable for production manufacturing finishing lines. However, recent advances by Corning Incorporated have potentially enabled the use of such methods in production glass finishing applications. Laser scoring typically starts with a mechanical check placed at the edge of the glass. A laser with a shaped output beam is then run over the check to create an expansion to occur on the glass surface, followed by a coolant quench to put the surface in tension, thereby thermally propagating a crack across the glass in the path of travel of the laser. Such heating is a localized surface phenomenon. The coolant directed behind the laser causes a controlled splitting. Stress equilibrium in the glass arrests the depth of the crack from going all the way through, thereby resulting in "score-like" continuous crack, absent of lateral cracking.
Unfortunately, laser scored edges alone are not as durable as beveled "ground" edges, due to the sharp edges produced during the laser scoring process. It would therefore be desirable to design processes to remove such sharp edges without having to resort to the mechanical diamond wheel like grinding methods traditionally employed.